Optical memory systems have been recently used for recording and reproducing large amounts of data. These systems include devices such as a magneto-optical memory system capable of recording and reproducing data in a magneto-optical recording medium such as a magneto-optical disk, magneto-optical card and others.
The magneto-optical recording medium comprises a disk-shaped or a card-shaped substrate made of a plastic or glass and a magnetic film made of Gd, Te, Fe, Co, or the like formed on the back face of the substrate. The direction of magnetization of the magnetic film changes according to an external magnetic field which exerts as influence upto the magnetic film, when the temperature rises to Curie point or higher.
With reference to FIGS. 6 and 7, there is shown a data recording operation using such a magneto-optical memory system. The magneto-optical disk 11 and the magneto-optical card 16 each have a magnetic film whose magnetizing direction is prefixed in a desired uniform direction as the device is rotated or slid. The effect of the external magnetic field generated by the magnet 12 or magnet 17 is applied to the device in the opposite direction to the magnetizing direction of the magnetic film.
At the same time, a light beam whose intensity varies in accordance with data to be recorded is converged by an objective lens 13 or objective lens 18 so as to have a diameter of approximately 1.mu.m and is projected onto the magneto-optical disk 11 or the magneto-optical card 16.
In the case where the light beam has a high intensity, the temperature locally rises in the area irradiated by the light beam on the magnetic film of the magneto-optical disk 11 or the magneto-optical card 16. When this temperature increases higher than Curie point, the direction of magnetization of the magnetic film is inverted due to the effect of the external magnetic field generated by the magnet 12 or the magnet 17. Data recording can be performed by the above inversion of the magnetizing direction of the magnetic film of the magneto-optical disk 11.
On the other hand, when reproducing data from the magneto-optical disk 11, a light beam which is linearly polarized and has such intensity that the temperature at the magnetic film does not rise higher than Curie point is projected onto the magneto-optical disk 11, and specifically in the area where data has been recorded is traced. The polarization plane of reflected light from the magneto-optical disk 11 is inclined in compliance with the magentizing direction of the magnetic film at the area irradiated by the light beam due to magneto-optical effects such as Faraday effect and Kerr effect. The incline of the polarization plane of the reflected light is converted into an electric signal by a photo-detector by way of an analyzer, thereby reproducing recorded data.
When performing data recording or data reproduction as described above, it is required that the light beam be projected onto an appropriate position of the magneto-optical disk 11. Therefore, as shown in FIGS. 8 and 9, the magneto-optical disk 11 and the magneto-optical card 16 are respectively provided with guide grooves 11a and guide grooves 16a formed in a direction parallel to the relative moving direction of the light beam and the magneto-optical disk 11 or the magneto-optical card 16.
More specifically, when the light beam is projected in the vicinity of an edge protion of the guide grooves 11a, the intensity of the reflected light decreases owing to the effect of diffraction or the like. This decrease in the intensity of the reflected light is converted into an electric signal and detected separately from signals to be obtained by the aforementioned magneto-optical effects using the photo-detector, thereby judging whether the irradiating position of the light beam on the magneto-optical disk 11 is appropriate or not. Thereafter, on the basis of the signal thus detected, the irradiating position of the light beam is controlled whereby decentering and incline caused by installment of the magneto-optical disk 11 on a rotation axis or a slide stand can be cancelled.
As shown in FIGS. 10 and 11, an identification (ID) section 11b is disposed at a part of each guide groove 11a and an ID section 16b at a part of each guide groove 16a in order to identify on which groove the light beam is projected. Each ID section 11b has pits and when the light beam is projected on the ID sections 11b, the intensity of the reflected light changes due to diffraction or the like which occurs in accordance with the pit pattern. By detecting the change in the reflected light intensity and reproducing an ID signal, it can be identified on which guide groove 11a the light beam is projected. To project the light beam on a desired guide groove 11a, an access operation is performed by moving the light beam in a direction perpendicular to the direction that the guide grooves 11a extend. In this case, an ID signal cannot be reproduced during movement of the light beam, so that the travelling distance of the light beam to the desired guide groove 11a is estimated and a coarse access operation is carried out by moving the light beam the estimated distance.
Thereafter, the light beam is shifted in a nearest guide groove 11a where an ID signal is reproduced thereby judging whether the guide groove 11a on which the light beam is projected is the desired guide groove or not. Then, the light beam is gradually moved and the above discussed judging operation is repeated until the ID signal at the desired guide groove 11a is reproduced whereby the light beam can be projected on the desired guide groove 11a.
In the foregoing coarse access operation, the pitches of the guide grooves 11a and 16a are of the order of .mu.m, whereas it is quite difficult to increase the mechanical accuracy of the system for moving the light beam to the same level as the above.
In order to overcome such a problem, the following arrangement has been conventionally proposed as shown in FIGS. 12 and 16. That is, reflected light from the magneto-optical disk 11 is guided into a photo-detector 22 via a half mirror 21 and converted into an electric signal. The electric signal thus obtained is converted into a binary signal with a threshold voltage Vt thereby obtaining a pulse signal. By using a so-called track count method in which the number of pulses in a pulse signal is counted by a digital counter (not shown), the number of guide grooves 11a through which the light beam has passed, i.e., the moving amount of the light beam can be obtained. Generally, the rotation or sliding of the magneto-optical disk 11 or the magneto-optical card 16 is continuously carried out during the access operation for projecting the light beam on a desired guide groove 11a, 16a. Therefore, the light beam sometimes obliquely crosses the guide grooves 11a, 16a passing through the ID sections 11b, 16b.
In this case, there exists in the output signal from the photo-detector 22, an ID signal or the like based on the pit pattern formed in the ID section 11b as shown in FIGS. 13 and 17, and therefore, the number of pulses in the pulse signal generated by a comparator 23 is not coincident with the number of guide grooves 11a through which a light beam 14 has actually passed.
In the conventional optical memory system as above described, the number of pulses in the pulse signal from the comparator 23 is included in the number of guide grooves 11a through which the light beam acutually has passed. Consequently, the moving amount of the light beam cannot be precisely obtained, and this results in low accuracy of the coarse access operation.